The invention relates generally to immersion pumps, and more particularly to an immersion pump for fluids having low-boiling points in which the rotational support and over-all construction of the immersion pump is virtually maintenance free.
Fluids having low-boiling points are generally in equilibrium with their vapors, especially when stored in a vessel or supply tank. For economically decanting low-boiling fluids, such as, for example, liquid helium or liquid hydrogen, from a larger supply tank into a smaller transport vessel, it is necessarY to return the cold gas, which is displaced from the transport vessel by the transferred liquid, back into the supply tank. If this is accomplished a considerable amount of losses in the form of cold gas can be saved. For the application of such a transfer procedure a pump is necessary which moves the fluid into the transport vessel with increased pressure through a first transfer line, while allowing vapor to escape from the transport vessel and to flow back into the supply tank through a second transfer line.
Additionally, since a low-boiling fluid in a supply tank is in equilibrium with its vapor, a pump for transporting such a fluid can not be positioned above the liquid, thereby creating a suction height, since vapor can enter the pump and the fluid flow. The transport pump must at least dip into the fluid and is preferably inserted into the tank and the liquid as deeply as possible.
In general, no large openings exist in storage tanks that would allow the insertion and installation of a pump for low-boiling fluids. Therefore, such pumps are typically mounted within the sealed storage tank and must be small in size while still providing high feed performance. Furthermore, because of the relatively complicated installation of the pump within the supply tank, it is desirable to provide a pump capable of long-term operation that is as unrestricted and as free from maintenance as possible.
In addition, the heat losses generated by such pumps must be kept as low as possible. Since the evaporation temperatures of the low-boiling fluids, especially of helium, are low, any heat produced by the pump leads to a considerable formation of vapor that in turn relates to a loss of fluid. Also, the appearance of vapor bubbles in the conveying fluid current reduces the pressure produced by the pump and must therefore be avoided. Accordingly, as close to a single-phase fluid current as possible should be maintained. This makes it necessary to provide a particular type of sealing Of the inlet and the outlet of the pump impeller wheel and a particular type of vapor control system.
A rotary pump for liquid helium was disclosed in July 1975, in a final report of the U.S. Department of Commerce, "Performance Characteristics of a Liquid Helium Pump". That report relates to an impeller wheel which is open on one side, is positioned in a suspended manner, and sits on a common shaft with the rotor of an induction motor. The shaft is supported with ball bearings, which, like the entire rotor, operate in liquid helium. These ball bearings, however, are not usable at low temperatures, especially at the temperatures of liquid helium or liquid hydrogen, unless they are specially constructed with bearing cages having special self-lubricating materials. The performance of these types of ball bearing structures in such low-boiling fluids, however, is not at all comparable with the performance which can be attained by these ball bearing structures at room temperature. This is due to the fact that friction losses inherent in these ball bearing structures at low temperatures are considerably higher than when they are operated with normal lubrication at room temperature. Thus, the requirements for unrestricted, maintenance-free, long-term operation, including reduced friction, can not be fulfilled by means of these known liquid helium rotary pumps.
It is also known to provide a magnetic support for a rapidly-operating shaft of a turbine rotor as described in German Patent No. DE-PS 21 37 850. In this arrangement, special structure is provided for the radial centering and axial positioning of the rotor. The centering takes place by utilizing ring-shaped cutting pole shoes which are positioned on the ends of the rotor shafts and are used in conjunction with permanent magnet systems. The axial support of the shaft takes place by utilizing active, controlled, magnet systems which engage separate, disk-shaped pole shoes positioned on the shaft. The signals necessary for the control of the axial magnets are obtained through an optical scanning of the shaft position. The signals, once electronically processed, are conveyed to the axial magnet systems. The magnet systems provide an axial restoration force with damping to the shaft, which overcomes the negative axial restoration forces of the centering systems. The optical scanning takes place at a single point on the ring-shaped cutting pole shoes of one of the ends of the shaft. Since such an optical scanning can produce signals, even during lateral movements of the shaft, it is also suitable for combatting parasitic forms of movements of the rotor, such as precession, nutation, and rolling movements. In this situation, the pole shoes of the axial magnet system standing opposite the disk-shaped pole shoes on the shafts obtain a slight inclination position and become skewed. Such magnetic supports are not suitable for use in an immersion pump since they are positioned close to the point of the optical scanning. Therefore, if immersed in a fluid, disturbances from the formation of bubbles and possibly through occasional deposition of contaminants as well can not be avoided and can affect the positioning of the shaft.